Conventionally, there has been proposed a radio communication system in which a plurality of antennas is arranged in a cell formed by a base station in order to improve the communication quality between the base station and mobile stations. For example, Long Term Evolution (LTE) defines a method for arranging a plurality of antennas at different positions in the same cell and a method for forming an individual cell for each antenna. In a radio communication system applied with the former method, an antenna of a base station forming a cell and a plurality of antennas both exist in one cell, but overhead of a handover process is reduced. When a plurality of antennas exists in the same cell as described for the former method, a base station assigns CSI-RSs (Channel State Information-Reference Signal) of different sequence numbers to the respective antennas. Thus, the base station can receive a report of an appropriate CSI value from a mobile station.    Non Patent Document 1: 3GPP TS36.211 V10.2.0(2011-06)    Non Patent Document 2: 3GPP TR36.814 V9.0.0(2010-03)
However, the CSI-RS has limitation on the number of ports that determines physical resources available for assignment (for example, eight ports per one cell). Especially in a radio communication system having many antennas arranged in one cell, it is highly possible that the number of ports to be assigned to the antennas is not enough. In order to solve this concern, there is known a method for increasing physical resources (the number of ports per one cell). However, this method results in improvement of control channel regions requested for signal transmission. Then the increase causes decrease of data assignment regions for downlink data. As a result, capacity of data that can be transmitted to mobile stations by a base station decreases, and thus throughput of the radio communication system is deteriorated. Since increase of the number of antennas in a cell decreases the number of mobile stations per one antenna, resources available for assignment to mobile stations increase, which results in increase of throughput of the whole system. Therefore, the limitation on the number of ports for the CSI-RS has been a factor that inhibits improvement of throughput.